The advent of portable computers has significantly increased the demand for safe, small sized long active-life batteries, having flat discharge curves at adequate voltage levels. While the demand for more functions has increased power requirements, the size of portable computers is ever decreasing, thereby requiring batteries to have increased energy and power densities.
Two types of batteries are used in personal computers. The first type provides primary power for the entire computer and the second type provides a back-up power source for the real time clock and other functions. In desktop computers, the back-up power source is required only for the real time clock and configuration memory retention, while in portable computers back-up power is additionally required for microprocessor suspend functions which increase energy drain rates. Consequently, since the increase in energy drain rates on an intermittent basis. As a result, back-up power sources in portable computers must be capable of providing more power on an intermittent basis than their desktop computer counterparts.
At first impression nickel-cadmium rechargeable batteries might appear to be suitable for providing back-up power in portable computers. However, sporadic use of portable computers often yields insufficient charge time for nickel-cadmium batteries, resulting in an increase incidence of data loss and battery failure. Additionally, nickel-cadmium batteries self discharge at a rate of 1% per day. Thus, such batteries may be totally discharged prior to purchase by the consumer. Moreover, cadmium is considered an environmental hazard rendering the disposal of nickel-cadmium batteries problematic.
Other battery systems are similarly inappropriate for use as back-up power sources. For example, lithium thionyl chloride batteries feature a low voltage, i.e., 3.6 Volts, close to the minimum voltage required to power most integrated circuits (hereinafter "IC" or "lCs"). Additionally, lithium thionyl chloride cells have been shown to be extremely dangerous in accidental abuse situations. Safer lithium chemistry cells such as lithium manganese dioxide button cells feature even lower voltages of 3.0 Volts or less, thus further limiting IC selection. Moreover, the relatively large size of such cells may cause problems when attempting to design low profile devices.
Another possible solution is to use metal-air cells, i.e., electrochemical cells wherein the oxygen in the air is the cathode material and a metallic material is the anode material. In many instances the preferred metal is zinc. In metal-air cells, air enters the cell through one or more pods in the cell which are either immediately adjacent to a cathode assembly, or separated from the cathode assembly by an air chamber. In either arrangement, the air diffuses into the cathode assembly where the oxygen in the air reacts with the water in the electrolyte consuming electrons and producing hydroxide ions. These ions then oxidize the metallic anode material producing one or more electrons for each atom of metal reacted. Such air cathode electrochemical cells are well known, and are more fully discussed in U.S. Pat. No. 4,591,539 (Oltman et al.). Moreover, metal-air cells am environmentally benign.
The cathode assembly also decreases the rate of diffusion of other gases into and out of the cell, particularly water vapor. The rate of oxygen ingress into the cathode to react with water and thereby produce hydroxide ions is the limiting factor to the rate of discharge of the cell. The moisture content inside the cell is balanced with the metal (e.g., zinc) content, for the most efficient use of this anode material. The gain or loss of too much moisture can reduce discharge efficiency.
Some prior ad metal-air cells provided an air chamber between the bottom side of the restrictive membrane and the interior surface of the bottom of the cathode can. See, e.g., U.S. Pat. No. 4,404,266 (Smilanich). Such prior art air chambers enable air to diffuse away from the air pods in the cathode can and react uniformly with the entire surface of the cathode assembly, instead of reacting disproportionately with those potions of the cathode assembly nearest to the air ports.
To limit water loss/gain between the cell and its immediate environment, some metal-air cells include a restrictive membrane between the exterior of the cell and the cathode layer. The restrictive membrane is usually located immediately adjacent to the cathode assembly. In other words, in prior art cells having an air chamber, the restrictive membrane is located between the air chamber and the air cathode assembly. Regardless of whether the cells provide an air chamber, the restrictive membrane limits moisture transport into and out of the cell. As noted above, the moisture content of the cell interior is one of the factors influencing cell efficiency.
A restrictive membrane limits not only the flow of moisture into and out of the cell, but also the flow of oxygen into the cell. Because oxygen is the cathodic material in metal-air cells, a reduction in oxygen ingress rate limits the rate capability of such cells. Because portable computers may require that the backup power source provide a pulse capability, prior art low rate metal-air cells have not been preferred for such applications.
However, due to their non-toxic chemistry, the relatively flat discharge curves they produce, and their long life, metal air batteries would be suitable candidates for back-up power sources in portable computers if, for example, oxygen ingress and water loss rates could be controlled to permit a high pulse capability.
Consequently, it is an object of the present invention to provide a battery having a long life under low discharge conditions.
Another object of the present invention is to provide a long-life battery having increased pulse capability.
Still another object of the present invention is to provide a long-life battery which produces a voltage greater than the IC effective minimum of three volts, yet has a significant pulse capability.
Yet another object of the present invention is to provide for a long-life battery that occupies a minimum amount of space.
It is another object of the present invention to provide a battery that does not present a hazard to the environment.
It is yet another object of the present invention to provide a battery suitable for low-discharge rate back-up power applications in computers.
Another object of the present invention is to provide a metal-air cell which controls air ingress such that the cell can provide pulse currents.
Still another object of the present invention is to provide metal-air cells which are substantially unaffected by changes in the ambient relative humidity.
These and other objects of the present invention, as will become more readily apparent hereinafter, are achieved by the invention described herein below.